The typical commercially used disposable article adhesive uses a plasticizing oil, a tackifying resin in combination with typically a styrene-butadiene-styrene (S--B--S) or styrene-isoprene-styrene (S--I--S) linear (A--B--A) block or a multi-block (A--B--A--B--A--B) copolymer having a molecular weight less than about 140,000. Such conventional commercial adhesives are typically formulated to contain about 20 wt-% S-I-S or S-B-S, A-B-A block copolymer, preferably 25 to 35 wt-% with 40 to 60 wt-% tackifier and 25 to 35 wt-% oil. Amounts of A-B-A block copolymer ranging from 20-35 wt-% of the adhesive are required to maintain cohesive strength and adhesion and to enhance the creep resistance properties in linear A-B-A block copolymer systems when used in the elastic attachment mode. The use of this amount of A-B-A rubber in these formulas result in high costs when compared to other adhesive systems. Additionally, such an adhesive with a high polymer content can result in a product viscosity that is in excess of the maximum viscosity used in the multi-line article construction. Such an adhesive with a lower polymer level (i.e. about 20 wt-%) that has a viscosity in the multi-line article construction range can lose cohesive strength, adhesion or creep resistance and can often fail in the elastic attachment mode.
Disposable articles and their manufacture from construction materials including woven and nonwoven fabrics, films and adhesives are described in a variety of U.S. patents. Buell, U.S. Pat. No. 3,860,003, Woon et al, U.S. Pat. No. 4,050,462, Strickland et al, U.S. Pat. No. 4,253,461, and Ryan et al, U.S. Pat. No. 4,326,528 teach disposable diaper construction. Such diapers typically comprise a fluid impervious backing sheet and absorbent materials applied to the backing sheet. Such diapers typically also comprise an elastic band attached to the portion of the diaper forming the leg opening. The absorbent material and the elastic bands are attached to the poly impervious backing sheet with hot melt adhesives.
Puletti et al, U.S. Pat. No. 4,419,494 teaches the use of S--B--S linear A--B--A block copolymers in conjunction with a fatty acid polyamide in heat resistant hot melt adhesives for use in the elastic attachment mode. Roeder, U.S. Pat. No. 3,672,371, Allison, U.S. Pat. No. 4,531,945, and VanTilberg, U.S. Pat. No. 4,687,478 teach various aspects of sanitary absorbent articles. Such articles typically comprise a fluid impervious back sheet with an absorbent layer attached thereto. Such absorbent articles can have a pervious top sheet over the absorbent layer and also can be fully wrapped by a pervious outer wrap. Such constructions are made using hot melt adhesives. Marsan et al, U.S. Pat. No. 4,392,862, Buell, U.S. Pat. No. 4,396,645 and Lindman et al, U.S. Pat. No. 4,681,793 teach a variety of absorbent devices and their materials and methods of construction. Collins et al, U.S. Pat. No. 4,136,699 teaches an A--B--A block copolymer based construction and positioning adhesive for absorbent articles.
Schmidt, Jr. et al, U.S. Pat. No. 4,526,577 is primarily directed to the use of styrene-butadiene-styrene linear A--B--A block and linear A--B--A--B--A--B multi-block copolymers in hot melt adhesives in the manufacture of disposable laminates using multi-line extrusion adhesive application technology. Schmidt, Jr. et al teach that hot melt adhesives have found a wide range of uses, however while a particular hot melt adhesive may have adequate bonding in a particular use, it may be completely unsuitable for other uses or applications. Of the many various hot melt adhesive compositions that have been proposed for use in construction of disposable articles, the use of a particular hot melt adhesive is dependent on its properties, particularly its ability to bond to polyethylene or polypropylene films, tissue, absorbent materials, and elastic banding.
The primary focus of the Schmidt patent is directed to bonding laminates with linear A--B--A and linear multi-block A--B--A--B--A--B block copolymers which are specifically disclosed and claimed in the patent. Schmidt does not appear to teach elastic band attachment. Only the multi-block A--B--A--B--A--B polymer based adhesives are exemplified in the patent. The patent discloses but does not claim radial or teleblock block copolymers and fails to disclose any aspect of such polymers including molecular weight, percent styrene, percent midblock, percent di-block, molecular weight distribution, or other polymer indicia relevant to adhesive properties and formulation. Further, Schmidt teaches that the adhesive material should contain 15 to 35 wt-% of the preferred linear or A--B--A--B--A--B multi-block copolymer and prefers and exemplifies adhesives containing 20 wt-% or more of the multi-block copolymer. Additionally, the Schmidt patent prefers relatively high concentrations of styrene, i.e. at levels of at least 35 wt-% or as much as 43 wt-% styrene in the most preferred A--B--A--B--A--B multi-block copolymer.
The typical S-E-B-S, S-B-S block copolymers identified by the Schmidt et al patent for the adhesives include linear A-B-A block copolymers which range in molecular weight from about 50,000 to about 140,000. The most preferred adhesives contain STEREON 840A, a multi-block A--B--A--B--A--B block copolymer having 35 wt-%, or greater, styrene and a molecular weight of about 70,000.
In conventional production of such composite disposable articles, hot melt adhesives are typically extruded at elevated temperatures (about 250.degree.-350.degree. F.) directly onto a work piece, typically a polyethylene or polypropylene film. Additional layers such as a nonwoven fabric, an absorbent material, or a film can be adhered to the poly film substrate using the hot melt adhesive. Spray adhesive technology has been studied and has attracted increasing attention in recent years. However, the predominant application technique mode in production remains the extruded fine line, multi-dot or multi-line methods.
In adhesive construction of disposable articles a variety of materials are bonded under a wide range of conditions. To optimize performance separate adhesives have evolved for use in the manufacture of premium quality disposable articles. This is particularly true in elastic attachment and in laminate construction. For the most part multi-line construction requires an adhesive with controllable, relatively low viscosity and sufficient bonding strength to maintain the mechanical integrity of the laminate composite article comprising a substrate and an absorbent layer. In sharp contrast to bond elastic materials to substrates, a different adhesive is currently used that primarily exhibits high creep resistance to ensure that the elastic, when under stress, does not move relative to the surface of the substrate or become partially or fully detached. Should the elastic move or become detached, for example in a disposable diaper, the resulting loss of fit could result in inconvenience, waste, embarrassment, discomfort, and associated health and safety problems.
The use of separate adhesives for fine line and for elastic attachment purposes, each having different formulas and properties, increases disposable article manufacturing complexity and can reduce productivity. Additionally, if multiple adhesives are required inventory and storage problems are increased. If the incorrect adhesive is used in the elastic banding it can lead to bond failure. Also, the multi-line extruder nozzle can become plugged. Such problems can lead to inferior products, lost production or both. While some disposable diapers are manufactured with a single adhesive for both multi-line construction and elastic attachment, those diapers are not premium diapers in that they often do not have premium mechanical integrity and can exhibit elastic creep, or detachment.
Accordingly a substantial need exists for a single adhesive having properties rendering the adhesive applicable to both multi-line construction and elastic bonding applications, which requires a significant advancement in providing improved bond strength, creep resistance and low application viscosity in a single adhesive.